Variable width waveguide for mode-matching and method for making

ABSTRACT

A variable width waveguide useful for mode matching between dissimilar optical waveguides and optical fibers and a method for making the same is described. In one embodiment, a tapered waveguide is etched in a substrate, a cladding material is laid over the upper surface of the substrate and within the waveguide, and the waveguide is then filled with a core material. The core material may be deposited in a single step, or in successive deposition steps.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from provisional applicationsserial Nos. 60/243,444, filed Oct. 26, 2000, and 60/249,793, filed Nov.16, 2000, the entire disclosures of which are incorporated herein byreference.

FIELD OF INVENTION

[0002] The invention relates to the manufacture of optical couplingdevices, and more particularly to the formation of optical couplingdevices having variable width waveguides.

BACKGROUND

[0003] Conventionally, optical fibers have waveguide modes which areshaped differently from integrated optic waveguides to which they areoptically connected. To provide an efficient optical coupling betweenthe optical fibers and the integrated optic waveguides, a mode converteris required. Known mode converters include tapered waveguides and GRIN(GRadient INdex) lenses. Examples of known tapered waveguides may befound in U.S. Pat. Nos. 5,854,868 (Yoshimura et al.), 5,265,177 (Cho etal.), and 5,009,475 (Knudson).

SUMMARY

[0004] The invention provides an optical waveguide that includes asubstrate having an upper surface and a trench extending therethrough, acladding material in the trench, and a core material in the trench. Thetrench has a varying profile along its length.

[0005] The invention further provides an integrated optic chip thatincludes a substrate having an upper surface, a waveguide having avarying profile along at least a part of its length, and integratedoptical circuits optically coupled to the waveguide. The waveguide has atrench extending through the substrate, a cladding material in thetrench, and a core material in the trench. The trench has a varyingprofile along at least a part of its length.

[0006] One aspect of the invention is a method for forming an opticalwaveguide that includes forming a trench through a substrate having anupper surface, wherein the trench has a varying profile along itslength; locating a cladding material in the trench; depositing a corematerial on the cladding material in the trench; and planarizing thesubstrate to the upper surface.

[0007] In another aspect of the invention, a method for forming anoptical waveguide includes the steps of forming a trench through a glasssubstrate having an upper surface, wherein the trench has a varyingprofile along its length; depositing a core material in the trench; andplanarizing the substrate to the upper surface.

[0008] In another aspect of the invention, a method for forming anoptical waveguide includes positioning a diffusion mask on an uppersurface of a substrate, the diffusion mask including a tapered middleportion; diffusing ions through the tapered middle portion to form awaveguide; and dipping the substrate in an ion solution causing ions todiffuse through the substrate and causing the waveguide to take on agenerally circular cross-sectional profile as it propagates into thesubstrate.

[0009] These and other advantages and features of the invention will bemore readily understood from the following detailed description of theinvention which is provided in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a top view of a substrate with a tapered trenchconstructed in accordance with an embodiment of the invention.

[0011] FIGS. 2-7 are side views of various stages of fabrication of atapered waveguide from the substrate of FIG. 1.

[0012]FIG. 8 is a top view of an integrated optical device constructedin accordance with another embodiment of the invention.

[0013]FIG. 9 is a top view of a multi-mode interference deviceconstructed in accordance with another embodiment of the invention.

[0014]FIG. 10 is a top view of a stepped waveguide constructed inaccordance with another embodiment of the invention.

[0015]FIG. 11 is a top view of a variable width waveguide constructed inaccordance with another embodiment of the invention.

[0016] FIGS. 12-15 are side views of various stages of fabrication of atapered waveguide from the substrate of FIG. 1 in accordance withanother embodiment of the invention.

[0017] FIGS. 16-17 are graphs illustrating the index of refractionacross a core of the waveguide of FIGS. 14-15.

[0018] FIGS. 18(a)-18(g) are graphs of examples of possible refractiveindex profiles at the large end of waveguides constructed in accordancewith an embodiment of the invention.

[0019] FIGS. 19-22 are side views of various stages of fabrication of atapered waveguide from the substrate of FIG. 1 in accordance withanother embodiment of the invention.

[0020]FIG. 23 is a top view of a masked substrate to be fabricated inaccordance with another embodiment of the invention.

[0021] FIGS. 24-25 are side views of the substrate of FIG. 23.

[0022]FIG. 26 is a side view of an optical device constructed inaccordance with an embodiment of the invention.

[0023]FIG. 27 is a side view of an optical device constructed inaccordance with another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] FIGS. 1-7 illustrate the fabrication of a tapered waveguidecoupler 10 (FIGS. 6-7). The fabrication of the tapered waveguide coupler10 begins with a substrate 12. The substrate 12 may be formed ofsilicon. A tapering trench 16 is formed in an upper surface 14 of thesubstrate 12. The trench 16 is tapered from a large end 18 to a smallend 20. The trench 16 may taper in thickness, width or both.

[0025] The tapering trench 16 may be formed through the use ofgray-scale masking combined with etch techniques. Specifically, thetrench 16 may be formed by gray-scale patterning of photoresist,transferring the gray-scale photoresist pattern onto the upper surface14, and then etching the gray-scale pattern. Alternatively, the trenchcan be formed by isotropic etching and agitation.

[0026] Next, a cladding material 22 is administered to the upper surface14 and the trench 16 of the substrate 12 (FIGS. 4-5). Preferably, thesilicon substrate 12 is oxidized with a thermal oxidation process.Alternatively, the substrate 12 may be covered with a chemical vapordeposition (CVD) oxide or spin-on glass. Also, a combination of thermaloxide and CVD oxide can be used.

[0027] Referring specifically to FIGS. 6-7, a waveguide core material 24is deposited within the trench 16 on top of the cladding material 22.The waveguide core material 24 may be a silicon oxynitride, a dopedglass or similar material. Through a chemical-mechanical planarizationprocess, the core material 24 is planarized to the upper surface 14,thereby finalizing the waveguide coupler 10. The waveguide coupler 10 isparticularly useful for coupling to an integrated optic waveguide whichis made according to a method disclosed in U.S. Provisional ApplicationSerial No. 60/240,805, filed on Oct. 16, 2000 by the same inventiveentity as the present application, the entire disclosure of which isenclosed herein by reference. Optionally, an upper cladding layer can bedeposited after planarization.

[0028]FIG. 8 illustrates a tapered waveguide which has been incorporatedwith a waveguide on an integrated optic chip 110. Specifically, thetapered trench 16 has been joined to a waveguide 116 of the integratedoptic chip 110, thereby becoming a part of the integrated optic chip110. Through such an integrated optic chip 110, light may enter thelarge end 18 of the tapered waveguide 16 and travel down the waveguide116 in a direction A to schematically shown integrated optical circuits117. As noted above, the tapered section 16 as well as the waveguide 116may be formed utilizing gray-scale masks and etching. Typically, thelarge end 18 of the tapered section 16 corresponds to a highultra-violet exposed region of the photoresist and the small end 20corresponds to a partially ultra-violet exposed region of thephotoresist. Gray-scale processing and etching of the waveguides 16, 116further allows three-dimensional patterning capabilities over the entireintegrated optic chip 110. Specifically, the waveguides 16, 116 can haveindependently varied thicknesses and widths.

[0029] The gray-scale processing and etching may also be utilized toform additional optical coupling devices, such as a multi-modeinterference (MMI) device 210 shown in FIG. 9. As illustrated, an inputwaveguide 214 may be patterned in a substrate 212. Further, outputwaveguide channels 216 _(a), 216 _(b), 216 _(c) may be formed adjoiningthe input waveguide 214. Alternatively, a stepped waveguide coupler 310may be formed through gray-scale processing and etching. As shown inFIG. 10, a stepped waveguide 316 may be patterned in a substrate 312 tocreate the stepped waveguide coupler 310. Alternatively, gray-scalemasking and etching techniques may be used to create periodicvarying-width waveguides 416 in a substrate 412, resulting in awaveguide coupler 410 with periodic-width variations, as shown in FIG.11.

[0030] Instead of depositing a single waveguide core material 24, asingle, continuously varied deposition step may be employed, e.g., CVDcontinuously varying the gas stoichiometry of core material in thetapered trench 16 may be performed to fabricate a waveguide couplingdevice. Such a method is illustrated in FIGS. 1-5 and 12-15. As shown inFIGS. 1-3, a substrate 12 has a trench 16 formed in it. The trench 16tapers from a large end 18 to a small end 20. A cladding material 22 isdeposited over an upper surface 14 and within the trench 16 (FIGS. 4-5).With reference to FIGS. 12-15, successive depositions or a continuousvaried deposition of waveguide core material 123 are performed. Thefirst deposited waveguide core material 123 has a high index ofrefraction. Each successive deposition of the waveguide core 123 has areduced index of refraction, or the varied deposition is such that thedeposition goes from a waveguide core 123 with a higher index ofrefraction to one with a lower index of refraction. The successivelayers of deposited core material 123 create a graded composition corelayer. Once the graded composition core layer has been frilly depositedthen the device is planarized, thereby creating a waveguide coupler 510(FIGS. 14-15). The planarization process planarizes to the upper surface14 of the substrate 12.

[0031] FIGS. 16-17 illustrate the index of refraction of the claddingmaterial 22 and the waveguide core material 123. FIG. 16 showsrefractive index at the small end 20 and FIG. 17 shows refractive indexat the large end 18 of the substrate 12. As noted, the index ofrefraction for the cladding material 22 is less than the index ofrefraction of the waveguide core material 123. Further, as is shown, theindex of refraction of the waveguide core material 123 is greater forthe first deposited of such material and lessens with each successivedeposition to the surface of the waveguide coupler 510. It should alsobe pointed out that the lowest index of refraction at the small end 20may be greater than the lowest index of refraction at the large end 18.Since the waveguide within the tapered trench 16 is tapered, thewaveguide core material 123 varies smoothly along the length of thewaveguide. Thus, the waveguide coupler 510 is enabled to efficientlycouple between waveguides having different refractive indices. Forexample, the waveguide coupler 510 may be used to couple optical fiberswith a high Δn (with Δn equal to the core n minus the cladding n), orfor high refractive index waveguides, for example, silicon oxynitridewaveguides. This can be accomplished by coupling the optical fiber tothe large end 18 (with a low core n) and coupling a high Δn waveguide tothe small end 20 (with a high core n).

[0032] The index of refraction profile of a waveguide core material 123may tale on many shapes other than the shapes indicated in FIGS. 16-17.FIGS. 18(a)-(g) provide several representative examples of potentialrefractive index profiles of a waveguide core material at the large end18 of a waveguide coupler.

[0033] An alternative embodiment of the invention utilizes a glasssubstrate 512 (FIGS. 19-22) instead of the silicon substrate 12 (FIGS.1-7). As with the embodiment illustrated in FIGS. 1-7, a tapered trenchis formed through the glass substrate 512. The tapered trench may beformed by an isotropic wet etching, an anisotropic dry etching, or grayscale processing, for example. After the formation of the taperedtrench, a waveguide core material 124 is deposited within the trench.The glass substrate 512 can itself serve as the cladding. After thetrench is filled with the waveguide core material 124, the substrate 512is planarized as described above in previous embodiments. In theembodiment using a glass substrate, the waveguide can have any of therefractive index profiles shown in FIGS. 18(a)-18(g).

[0034] In an alternative embodiment illustrated in FIGS. 23-25, awaveguide may be formed in glass by the diffusion of ions from a saltmelt. A diffusion mask having a tapered shape is patterned over theglass substrate 512. The diffusion mask leaves unmasked a tapered middleportion 316 sandwiched between masked portions 318. Diffusion of ionsfrom the salt melt is allowed only in the middle portion 316 of themask. Further, the tapered shape of the unmasked area 316 allows moreions from the salt melt to diffuse into the glass at the large end ofthe waveguide than at the small end of the waveguide. The diffusion ofions through the unmasked middle portion 316 of the glass substrate 512creates a waveguide core material 125 as shown in FIGS. 24-25. Then, asdescribed in reference to FIGS. 21-22, the glass substrate 512 is thendipped into an ion solution. Ions diffuse through the glass substrate512 so that the waveguide propagates into the glass and becomes morerounded as it becomes further buried within the substrate 512.

[0035]FIG. 26 illustrates an optical device 700 which includes anoptical fiber 702 and an integrated optic chip 704 coupled together. Thefiber 702 is generally formed of a material which has a low Δn. Theoptic chip 704 includes a tapered waveguide portion 706 which iscontiguous with another waveguide portion 708. The tapered waveguideportion 706 couples with the fiber 702 at a large end 710, with a smallend of the waveguide portion 708 being on an opposite end of the opticchip 704. The waveguide portion 708 has a high Δn, and has a core with ahigher n than that of the fiber 702.

[0036]FIG. 27 illustrates an optical device 750 which includes the lowΔn optical fiber 702 and the integrated optic chip 704 coupled with asecond integrated optic chip 714. The small end 712 of the waveguideportion 708 couples to a second high Δn waveguide portion 716. Thewaveguide portion 716 has a core with a higher n than that of the fiber702.

[0037] While the invention has been described in detail in connectionwith exemplary embodiments known at the time, it should be readilyunderstood that the invention is not limited to such disclosedembodiments. Rather, the invention can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described, but which are commensurate withthe spirit and scope of the invention. Accordingly, the invention is notto be seen as limited by the foregoing description, but is only limitedby the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An optical waveguide, comprising: a substratehaving an upper surface and a trench extending therethrough, whereinsaid trench has a varying profile along its length; a core material insaid trench; and a cladding material in said trench between saidsubstrate and said core material.
 2. The optical waveguide of claim 1,wherein said trench has a profile which tapers from a large end to asmall end.
 3. The optical waveguide of claim 2, wherein said trenchtapers in thickness.
 4. The optical waveguide of claim 2, wherein saidtrench tapers from a large width to a small width.
 5. The opticalwaveguide of claim 2, wherein said core material at said small end has agreater index of refraction at said upper surface than said corematerial at said large end.
 6. The optical waveguide of claim 1, whereinsaid trench includes an input waveguide channel and a plurality ofoutput waveguide channels.
 7. The optical waveguide of claim 1, whereinsaid trench has a stepped profile.
 8. The optical waveguide of claim 1,wherein said trench has a profile with periodic variations in width. 9.The optical waveguide of claim 1, wherein said core material closest tosaid cladding material has a higher index of refraction than said corematerial located further from said cladding material.
 10. The opticalwaveguide of claim 1, wherein said substrate comprises silicon.
 11. Theoptical waveguide of claim 10, wherein said cladding material comprisessilicon dioxide.
 12. The optical waveguide of claim 1, wherein said corematerial comprises silicon oxynitride.
 13. The optical waveguide ofclaim 1, wherein said core material comprises doped glass.
 14. Theoptical waveguide of claim 1, wherein said substrate comprises glass.15. The optical waveguide of claim 14, wherein said core materialcomprises successive depositions of core material on said claddingmaterial, wherein each successive deposited core material has a lowerindex of refraction than the preceding deposited core material.
 16. Theoptical waveguide of claim 14, wherein said core material is buriedwithin said substrate and has a generally circular cross-section.
 17. Anintegrated optic chip, comprising: a substrate having an upper surface;a waveguide including: a trench extending through said substrate, saidtrench having a varying profile along at least a part of its length; acore material in said trench; and a cladding material in said trenchbetween said substrate and said core material; and integrated opticalcircuits optically coupled to said waveguide.
 18. The integrated opticchip of claim 17, wherein said trench has a profile which tapers from alarge end to a small end.
 19. The integrated optic chip of claim 18,wherein said trench tapers in thickness.
 20. The integrated optic chipof claim 18, wherein said trench tapers from a large width to a smallwidth.
 21. The integrated optic chip of claim 18, wherein said corematerial at said small end has a greater index of refraction at saidupper surface than said core material at said large end.
 22. Theintegrated optic chip of claim 17, wherein said core material closest tosaid cladding material has a higher index of refraction than said corematerial located further from said cladding material.
 23. A method forforming an optical waveguide, said method comprising: forming a trenchthrough a substrate having an upper surface, wherein the trench has avarying profile along its length; locating a cladding material in thetrench; depositing a core material on the cladding material in thetrench; and planarizing the substrate to the upper surface.
 24. Themethod of claim 23, wherein said forming comprises: gray-scalepatterning of a photoresist; transferring the patterning of thephotoresist to the substrate surface; and etching the patterning of thephotoresist.
 25. The method of claim 23, wherein said forming comprisesisotropic etching.
 26. The method of claim 23, wherein the substratecomprises silicon, said locating comprising oxidizing the claddingmaterial.
 27. The method of claim 26, wherein said oxidizing comprises athermal oxidation process.
 28. The method of claim 23, wherein saidlocating comprises covering said upper surface and said trench with achemical vapor deposition oxide.
 29. The method of claim 23, whereinsaid locating comprises covering said upper surface and said trench withan oxynitride.
 30. The method of claim 23, wherein said locatingcomprises covering said upper surface and said trench with a dopedoxide.
 31. The method of claim 23, wherein said depositing comprisessuccessive depositions of the core material on the cladding material,wherein each successive deposited core material has a lower index ofrefraction than the preceding deposited core material.
 32. The method ofclaim 23, wherein said planarizing comprises chemical-mechanicalplanarizing.
 33. A method for forming an optical waveguide, said methodcomprising: forming a trench through a glass substrate having an uppersurface, wherein the trench has a varying profile along its length;depositing a core material in the trench; and planarizing the substrateto the upper surface.
 34. The method of claim 33, wherein said formingcomprising isotropic wet etching of said substrate.
 35. The method ofclaim 33, wherein said forming comprising anisotropic dry etching ofsaid substrate.
 36. An optical device, comprising: an optical fiber; andan integrated optic chip including a tapered waveguide coupled to saidoptical fiber and an integrated waveguide portion coupled with saidtapered waveguide, wherein said integrated waveguide portion has a corewith a higher n than said optical fiber.
 37. The optical device of claim36, wherein said integrated waveguide has a higher Δn than that of saidoptical fiber.
 38. The optical device of claim 36, further comprising asecond integrated optic chip including a second waveguide portioncoupled with said integrated waveguide portion.